This invention relates to a method of melt-cutting workpieces using laser beams as a heat source and particularly to a method of cutting while keeping high cutting speed to provide a smooth cut surface.
In laser beam cutting, a workpiece is melted using laser beams along an intended area and a gas is blown against the workpiece to remove the melted portion.
In order to obtain a smooth cut surface, trials have been made to oscillate the focal point of laser beams in the direction of the optical axis (that is, the direction of the depth of cut), as disclosed in the following documents:
1) Takaes et.al., Advanced Laser Beam Cutting Using Adaptive Optics, ECLAT""96, pp971
2) M. Geiger, S. Schuberth and J. Hutfless, xe2x80x9cCo2 laser beam sawing of thick sheet metal with adaptive opticsxe2x80x9d, Welding in the world/Le Soudage dans le Monte, Vol.37, No.1,5 (1966)
In an ordinary laser cutting in which continuously oscillated laser beams are shed on a workpiece with the focal point fixed, streaks are formed on the cut surface due to interaction between the beam feed speed (or cut speed) and the oxidation reaction between oxygen and iron at the cut point. The streaks determine the roughness of the cut surface. The thus formed streaks are arranged at irregular pitches or intervals and thus roughen the cut surface.
Pulse oscillated laser beams are also sometimes used for laser cutting. Pulse oscillated laser beams minimize the heat-affected layer, thus permitting more precise cutting. But the cut speed using such laser beams is less than half the speed when continuously oscillated laser beams are used.
We therefore thought of developing the method proposed in the above-mentioned documents. That is, we thought that it might be possible to obtain a smoother cut surface without lowering the cutting speed if a workpiece is cut using continuously oscillated laser beams while oscillating the focal point.
But depending upon the conditions as to how the focal point is oscillated, this method can even produce a contrary result.
In this respect, in documents 1) and 2), the focal point is oscillated at frequencies of up to 150 Hz and 100 Hz, respectively. But at frequencies less than 150 Hz, the cut surface smoothing effect is not sufficiently obtained. Rather, the cut surface may be even rougher than in case the focal point is not oscillated.
We have also found out that in order to obtain a smooth cut surface, the amplitude of focal point oscillation is also an important factor affecting roughness of the cut surface and that it is equally important to minimize blurring of the condensation spot (or increase in spot diameter). The present invention has been made on basis of these findings.
An object of the invention is to provide a laser cutting method that offers a smooth cut surface without lowering the cutting speed and increases the smoothness to a practically acceptable level.
According to the present invention, a workpiece is cut while oscillating the focal point of laser beam in the direction of optical axis. To achieve the effect (that is, improved cut surface quality) by oscillation of focal point sufficiently and securely, the oscillation frequency should be adjusted to a range from 150 Hz to 300 Hz and the oscillation amplitude adjusted to a range between xc2x10.5 mm and xc2x13 mm.
To make possible oscillation at such a high frequency, a piezo-actuator is used to resiliently deform the mirror plate to change the curvature of the reflecting surface of the mirror plate, thereby controlling the divergence angle of laser beam incident on a condensing lens.
Further, if the surface accuracy of the reflecting surface of the mirror plate resiliently deformed for focal point oscillation is poor, the condensation spot is blurred, the power density of the laser beam lowers, and the cutting speed lowers. Thus, blurring of the condensation spot is suppressed by using a variable-curvature mirror having a mirror plate having a thickness distribution on its back or one in which the thickness is thicker at the central side.
Besides, for laser beams, continuous oscillation beams are used.
As mentioned earlier, natural streaks created on the cut surface in ordinary laser cutting are arranged at irregular pitches. In contrast, if cutting is performed while oscillating the laser beam focal point in the optical axis direction, it is possible to forcibly form on the cut surface regular streaks corresponding to the driving pitch of the focal point. If the frequency of focal point oscillation is controlled so that the pitch of the streaks is finer than the pitch of natural streaks to produce regular, fine-pitched streaks, the surface roughness improves, so that the cut surface is smoothened. It was found out that if the frequency of focal point oscillation is within the range of 150 Hz-300 Hz, the effect of smoothening of the cut surface will appear markedly, and that the effect will be the maximum especially at around 200 Hz. It was also found out that for flattening of the cut surface, it is necessary to keep the oscillation width or amplitude of the focal point within the range of xc2x10.5 mm to xc2x13 mm.
Next, as a method of oscillating the laser beam focal point at high speed in the optical axis direction, it is the most practical to use a laser machining machine equipped with a variable-curvature mirror in the optics and control the divergence angle of the beams incident to the condensation lens. This is because a piezo-actuator for resiliently deforming the mirror plate of the variable-curvature mirror is an element allowing shrink/expand control at high speed.
In order to prevent blurring of the condensation spot, the reflecting surface of the mirror should have a surface accuracy of less than {fraction (1/10)} of the wavelength. For CO2 laser beams (wavelength: 10.6 xcexcm), the reflecting surface accuracy has to be less than 1 xcexcm. One effective way to meet these requirements is to convex the back of the mirror because such a mirror is deflected uniformly when pressure is applied to the center of the back by the actuator. Even after deformation, the reflecting surface accuracy can be kept at 1 xcexcm or under.
As mentioned above, cutting speed using pulse laser beams is less than half the speed when continuous laser beams are used. Moreover, since the pulse oscillation frequency range substantially coincides with the focal point oscillation frequency range envisaged in the present invention (several tens of hertz to several hundreds of hertz), no expected advantages of the invention are sufficiently achievable with pulse oscillated laser beams. Thus, continuously oscillated laser beams are used.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: